1. Field of the Invention
The present invention relates to a light source apparatus for an electronic endoscope using a plurality of aperture-controlling rotary plates, and an electronic endoscope.
2. Description of the Prior Art
In a conventional electronic endoscope, in order to provide appropriate light modulation, an endoscope record apparatus has been proposed in, for example, Japanese Unexamined Patent Publication No. 62-69222. The apparatus disclosed in this publication is provided with a rotary shutter having a rotatable shaft, wherein the distance between the rotatable shaft and the axis of illumination light emitted from a light source for the endoscope is variable. The rotary shutter is shaped so that a difference in the peripheral speed occurs between the radial portions thereof or the aperture is varied in accordance with the rotation of the rotary shutter. The light modulation is carried out due to a change in the distance between the axes using the peripheral speed difference.
In Japanese Unexamined Patent Publication No. 62-69222, light modulation can be performed, however, the structure of the rotary shutter is complex. Furthermore, it is necessary to provide a mechanism to vary the distance between the rotary shutter and the optical axis of the light source for the endoscope. Accordingly, the manufacturing cost is high and the manufacturing process is troublesome. Moreover, in order to achieve such a construction, the outer diameter of the rotary shutter must be several times larger than that of the light bundle, thus resulting in an increase in the size of the rotary shutter. If the rotary shutter is asymmetrical in shape with respect to the rotation axis thereof in order to vary the aperture, the center of rotation does not align with the center of gravity, so that the rotary shutter tends to lose balance during rotation. Consequently, correct emission of the illumination light cannot take place and the rotary shutter and the surrounding members may break.
In view of the problems discussed above, the assignee of the present application has proposed a light source apparatus for an electronic endoscope having a light source and a rotary shutter whose rotation axis extends parallel with the optical axis of the light source and which intercepts or emits illumination light emitted from the light source toward a light guide. The rotary shutter is provided with a pair of aperture controlling rotary plates coaxial with each other, which are rotatable together and which are each provided with light intercepting portions and opening portions arranged alternately in the rotation direction, whereby the opening angle of the opening portions of the rotary shutter as a whole is varied by relatively rotating the aperture controlling rotary plates to thereby control the quantity of light to be emitted (Japanese Patent Application No. 2004-103941).
In the invention disclosed in Japanese patent Application No. 2004-103941, two motors are used to rotate the pair of aperture controlling rotary plates. One of the motors is stationary and the other motor rotates together with the aperture controlling rotary plate, and hence, providing a countermeasure to prevent interference of harnesses (wirings) extending from the motors.
Furthermore, if smooth rotation of the aperture controlling rotary plates does not occur for some reason, the illumination light may unintentionally flicker. If the opening area defined between the pair of aperture controlling rotary plates is large (if the brightness is high), the rotary shutter is less affected by the flickering of the illumination light (variation of brightness). However, if the opening area is small (if the brightness is low), the brightness of the illumination light is largely deviated from a desired value, and accordingly, the affection by the flickering of the illumination light is not negligible.
Moreover, if smooth rotation of the aperture controlling rotary plates does not occur, the rotation speed of the aperture controlling rotary plates (shutter speed) cannot be increased much.